A multi-antenna system, as a technology meeting the requirements of high speed, large capacity and reliable transmission for a mobile communication system, has been studied extensively in recent years. Studies have shown that, the multi-antenna system is able to provide a higher capacity as compared with a conventional single-antenna system, and the capacity of the multi-antenna system increases linearly with the increase of the number of antennas under certain conditions.
An antenna array is a multi-antenna system consisting of a plurality of identical antenna units arranged with certain regularity. The antenna array can be classified into a liner array, a planar array and the like according to the arrangement of antenna units. A common liner array is a rectilinear array where centers of antenna units are arranged equally spaced in a straight line. However, antenna units of the liner array may also be arranged unequally spaced, or centers of antenna units may not be arranged in a straight line, for example, centers of antenna units may be arranged in a circle. Multiple rectilinear arrays arranged on a certain plane at certain interval constitute a planar array. Multiple antenna units arranged with their centers on a spherical surface constitute a spherical array.
In a multi-antenna system, if a transmitting side can be informed of forward channel information in a certain manner, the transmitting side is able to optimize transmission signal according to forward channel characteristics, thereby improving the reception quality and reducing complexity of a receiving side. In an actual frequency division duplex (FDD) system, the forward channel information is conventionally fed back by quantizing the channel information in order to reduce feedback overhead and improve system transmission efficiency.
In an LTE (Long Term Evolved) frequency division duplex (FDD) system, a codebook-based implicit CSI (Channel State Information) feedback method is adopted. An UE (User Equipment, i.e., a terminal apparatus) measures downlink channel based on a pilot signal, and reports to a base station the number of data layers RI (Rank Indication) that can be supported by the downward channel and Precoding Matrix Indicator (PMI) information according to receiving and processing algorithm of its own. Further, the UE also needs to report Channel Quality Indicator (CQI) for each codeword.
The precoding related to PMI is a self-adaptive technology in the multi-antenna system. In this technology, a precoding matrix is adaptively changed by the transmitting side according to CSI, thereby changing the channel experienced by signals. A set of codebooks including several precoding matrices are stored in both the transmitting side and the receiving side, thus the receiving side may select one of the precoding matrices according to the estimated channel matrix and a certain criterion, and feedback an index value of the selected precoding matrix and the quantized channel status information to the transmitting side. At the next time, the transmitting side adopts a new precoding matrix and determines an encoding and modulation scheme for a codeword according to the quantized channel status information fed back.